DESCRIPTION Human and animal movement often uses energy more efficiently than expected from the efficiency of muscle alone. The extra efficiency often results from recovery of energy by elastic recoil of muscles and tendons after being stretched, Understanding such energy saving mechanisms requires examining the relationship between intracellular metabolism and extracellular mechanical output. Rattlesnake tailshaker muscle is an ideal model system for studying the relationship between energy supply and demand, including energy saving mechanisms, in muscle. Tailshaker muscle contracts at higher rates without fatigue; its single motor unit and simple all or none contractions facilitate experiments on physiology and mechanisms. This research will address (1) the force and mechanical work of muscle contraction. (2) The mechanisms for minimizing the cost of contraction, including elastic recycling, (3) Muscle efficiency with and without energy saving mechanisms. This work will employ force transducers for measuring muscles forces, sonomicrometry for measuring muscle length changes, and nuclear magnetic resonance spectroscopy for measuring muscle energetics in vivo. Together, these experiments will promote understanding of the relationships among muscle metabolism, biomechanics, and performance, as well as of muscle function during exercise, training, and recovery from injury or surgery.